1. Field of the Invention
The present invention relates to a cutting insert used in tooth cutting work for cutting tooth grooves of a gear or the like and an indexable tooth cutting tool using the cutting insert, in particular mainly for cutting an involute gear.
2. Description of the Related Art
A formed tooth cutting tool is used in tooth cutting work by a forming method of cutting out a tooth profile of a gear by an edge profile having the same contour as that of the tooth profile of the gear to be cut. In a conventionally known formed tooth cutting tool, cutting inserts are mechanically mounted on a plurality of insert mounting seats arranged along a circumferential direction in an outer peripheral portion of a substantially disc-shaped body rotated around an axis line.
The cutting insert attached to the indexable tooth cutting tool is typically a negative insert formed in a substantially square, planar shape. The insert is provided with a cutting edge in an intersection ridge line portion between a flank provided on a square face and a rake face provided on a side face perpendicular to the flank. The cutting edge is formed in an involute curved line shape corresponding to an involute tooth profile used in a majority of gears.
In a case where the cutting insert is mounted on an insert mounting seat, an axial rake angle is negative, thereby leading to poor cutting performance and high cutting resistance at cutting. Therefore, chattering vibrations tend to be easily generated, which raises a problem of inviting degradation of cutting accuracy, a reduction of tool lifetime due to damage of the cutting edge, and the like.
For overcoming this problem, in a cutting insert for a tooth cutter disclosed in Japanese Patent Laid-Open No. 2005-66780, as shown in FIG. 10A to FIG. 10C, an insert body 104 has a side face 104c which is formed to be gradually inclined to an inside of the insert body 104 from an upper face 104a of the insert body 104 toward a lower face 104b thereof, wherein intersection ridge lines between the side face 104c of the insert body 104 and the upper face 104a of the insert body 104 are defined as cutting edge ridges 105a and 105b for cutting tooth bottom and tooth plane portions of the involute gear.
In the invention according to Japanese Patent Laid-Open No. 2005-66780, the side face 104c of the insert body 104 as a rake face is composed of a flat plane inclined in a constant angle. However, when this inclination angle is excessively large, there is a possibility that the cutting edge ridge 105a for cutting tooth bottom portions of the involute gear located in a low position of an edge height from the lower face 104b to the cutting edge ridge 105 of the insert body 104 is biased inside of the insert body 104 in a plan view as seen from a direction opposing the upper face 104a of the insert body 104. On the other hand, there is a possibility that the cutting edge ridge 105b for cutting tooth profile portions of the involute gear located in a high position of the edge height is inflated outside of the insert body 104 in the plan view as seen from the direction opposing the upper face 104a of the insert body 104. Therefore, in a state where the insert body 104 is mounted on the tool body, a radial rake angle γf in the vicinity of the cutting edge ridge 105a for cutting the tooth bottom portion of the gear is excessively negative, and the cutting performance is deteriorated as compared to the cutting edge ridge 105b of the other portion. In consequence, this conventional type of insert has a problem that the inclination angle which can be applied to the side face 104c of the insert body 104 serving as a rake face is limited.